Vehicle cooling systems are known wherein a separate cooling unit is mounted in front of the radiator. For example, a four wheel drive tractors included an air-to-air charge air cooler which was mounted in front of the radiator. Such an arrangement is also shown in U.S. Pat. No. 5,316,079 issued to Hedeen on 31 May 1994. In such an arrangement the air flowing out of the upstream cooler can be heated to a temperature which is hotter than the coolant in the downstream radiator, and this can degrade the cooling function of the downstream radiator.